This invention relates to the production of hydrogen by steam reforming hydrocarbons. In another aspect it relates to a process for steam reforming methane using a particulate contact catalyst and an adsorbent in combination. In still another aspect it relates to apparatus suitable for steam reforming of methane to produce hydrogen.
The most common industrial process for the production of hydrogen involves steam-methane reforming (SMR) or steam reforming of naphtha. In the SMR process a gaseous mixture of steam and desulfurized light hydrocarbons (natural gas or naphtha) is fed to a reactor containing several tubes packed with particulate catalyst inside a reforming furnace which converts the feed material to a product of about 70 to 72 mole percent hydrogen, 6 to 8 percent unconverted methane, 8 to 10 percent carbon dioxide, and 10 to 14 percent carbon monoxide, all on a dry basis. This product stream is cooled and fed to another packed bed of catalyst for adiabatic conversion by the water-gas shift reaction (WGS) which increases the hydrogen content to 71-75 percent and the CO.sub.2 to 15-20 percent while reducing the methane content to 4-7 percent and the CO to 1-4 percent, all on a dry basis. Water present in this product is separated by condensation and normally the hydrogen is further purified by pressure swing adsorption (PSA). Although high purity hydrogen (98-99.999% H.sub.2) can be made by this process using PSA with a hydrogen recovery of 70-90 percent, the capital cost is very high because of the high temperatures (750-900.degree. C.) required in the SMR reactor, the heat recovery operations that are necessary, and the complex design of multistep, multicolumn PSA purification. Even though this process of H.sub.2 production is currently the best economic option, it is well recognized that reduction of this capital investment is highly desirable in order to lower the cost of hydrogen which is in great demand, particularly in refinery, space exploration, fuel cell application, and the chemical industry. Many attempts have been made in the past decade to solve this problem.
One of the difficulties involved in the above described process for H.sub.2 production is the reversible nature of SMR and WGS reactions which present equilibrium limitations on the purity of the product streams from the SMR and WGS reactors. In other processes limited by equilibrium in the principal reactions, efforts have been made to drive the reaction toward the product by separating one of the product components from the reaction system as soon as the product is formed. For example, Vaporciyan and Kadlec, "Equilibrium-Limited Periodic Separating Reactors", AlChE Journal, Vol. 33, No. 8 (1987) describe the use of a mixture of catalyst and adsorbent in a reactor operated like a single bed PSA operation for heterogeneous catalytic gas phase reactions. They discuss reaction-sorption equilibrium models on a theoretical level but do not apply the system to any particular process.
Westerterp et al., "Two New Methanol Converters", Hydrocarbon Processing, (Nov. 1988) describe a process for manufacture of methanol from synthesis gas wherein product as it is formed is removed from the catalyst surface by adsorption by continuously trickling adsorbent over the catalyst bed. This procedure is compared with an alternative process wherein conversion is carried out in a series of reactors with inter-stage product removal in adsorbers. Berty et al., "Beat the Equilibrium", Chemtech, (October 1990) describe the use of inert solvent to absorb methanol as soon as it is formed over a catalyst bed in a methanol synthesis process, thereby shifting reaction equilibrium toward the product. Mention is made of the alternative use of fine adsorbent powder to perform the same function.
Other simultaneous separation procedures have been suggested, such as the use of membrane separation of product from the reaction site in order to shift reaction equilibrium. Kikuchi et al. describe this approach for steam reforming of methane to produce hydrogen; see "Hydrogen Production from Methane Steam Reforming Assisted by Use of Membrane Reactor", Natural Gas Conversion, pp 509-515, Elsevier Science Publishers B. V., Amsterdam (1991).
Kirkby and Morgan of the Department of Chemical and Process Engineering, University of Surrey, Guildford, Surrey, England, in a paper titled "Pressure Swing Reaction--A Novel Process" for The 1991 Icheme Research Event, describe a PSA reactor as a new and untested device combining reaction and separation in a plant resembling a PSA system. Using a mathematical model for a single reactant "A" in an inert carrier gas reacted to form a product "B" which alone is adsorbable, the authors consider limitations on using some of the product stream as a purge stream after depressurization of the reactor and as a fluid to partially repressurize the reactor beds. Predictions are made that this procedure has a future in ethylene production because of its advantages over thermal cracking.
Adaptation of such a process for the dehydrogenation of cyclohexane was suggested by Goto et al. in "Dehydrogenation of Cyclohexane in a PSA Reactor Using a Hydrogen Occlusion Alloy", published in part at the 57th conference of the Chemical Engineering Society, Osaka, Japan, April 1992, Chemical Engineering Essays, Vol. 19, No. 6 (1992). Periodic regeneration of the adsorbent in simultaneous reaction-separation processes is described and the use of staggered phases employing multiple reactors to permit continuous operation is suggested.
U.S. Pat. No. 5,449,696, Dandekar et al., (1995) describes methanol production using a simulated moving bed of catalyst and adsorbent which separates methanol as it is formed from the reactants, H.sub.2 and CO. Desorption of methanol from the adsorbent is achieved by using CO.sub.2, H.sub.2 or methane at a temperature lower than the reaction temperature. Reactant desorbents are said to suppress back reaction of methanol and can be separated from the methanol and reused in the reaction. It is suggested that this process can be used in combination with other reactions such as desulfurization and reforming of methane with O.sub.2 to produce hydrogen.
Carvill et al., "Sorption-Enhanced Reaction Process", AlChE Journal, Vol. 42, No. 10 (1996) describe an improved process for equilibrium limited reactions for which they give the acronym "SERP". The process discussed uses simultaneous reaction and separation in a single operation with a mixture of catalyst and adsorbent in a fixed packed column in order to carry out a reverse water gas shift reaction for the production of CO. The process goes through the steps of (1) simultaneous catalytic reaction of CO.sub.2 with hydrogen and adsorption of product water with recovery of pure CO, (2) countercurrent depressurization, (3) countercurrent purge with a weakly adsorbed gas such as nitrogen, (4) countercurrent purge with CO, and (5) repressurization of the column with CO. The authors also discuss reaction and sorption dynamics within three zones in the reactor including two reaction mass transfer zones located at the feed and product ends of the column. It is also suggested that steam-methane reforming for the production of hydrogen is a candidate for SERP. Additional refinements in such processes are highly desirable to reduce product cost still further and improve the overall economy of the operations.